This invention is concerned with blind rivets.
Blind rivets are used in circumstances where access to both sides of materials to be joined is difficult or impossible. Such rivets comprise a rivet head, a shank projecting from one side of the rivet head, and force transmitting means which projects from an opposite side of the rivet head, extends through the rivet head and along the shank, and engages a portion of the shank. The force transmitting means is used, once the shank has been inserted through holes in two workpieces to be joined by the rivet and when the rivet head is held stationary against the first of the workpieces, to transmit force applied thereto on said opposite side of the rivet head, so that the force is applied to said portion of the shank. The force transmitted causes said portion of the shank to deform to form a further rivet head, which engages the second of the workpieces, thereby forming the completed rivet.
Most blind rivets are made of metal and their force transmitting means comprises a mandrel having a mandrel head, providing an anvil member to apply the force, and a stem connected to the mandrel head. The stem passes through a bore through the rivet head and the shank of the rivet. A line of the weakness is provided in the mandrel stem adjacent to the mandrel head allowing it to break off once the further rivet head has been formed. For example, blind rivets can be used to join the panels of an aircraft skin to the structural members of the aircraft. Where such blind rivets are made of metal, their performance is generally adequate, when joining metal workpieces. However, the use of metal blind rivets to join other materials can lead to problems. For example, composite materials made at least partially from polymeric materials, e.g. polyetheretherketone or polyphenylene sulphide filled with carbon fibre, are finding an increasing number of uses because of their strength and lightness. If metal blind rivets are used to join such materials, there is a thermal expansion mismatch between the material joined and the rivet with implications for tightness of the joint, in some cases a corrosion problem arises, and the presence of the metal increases the overall weight of the joined structure. For these reasons, such materials are normally secured by adhesive. However, such adhesively bonded joints generally fail prior to failure of the structural components which is contrary to design criteria of the aircraft industry, for example. Therefore, a requirement exits for a satisfactory blind rivet made a least partially of a polymeric material, such as a composite material of the type referred to above.
Blind rivets which are made of polymeric material, however, have shanks which are lacking in ductility when cold. This means that sufficient force cannot be transmitted by conventional means to the shank to form said further head.
It has been proposed for the portion of the shank to be deformed to comprise only partially cross-linked thermosetting polymeric material which is heated after insertion of the rivet by applying heat to a conventional mandrel (see U.S. Pat. No. 4,478,544). The material of the shank is softened by the heat and becomes sufficiently ductile to allow the further rivet head to be formed. Continued application of force and heat causes the material to crosslink so that the further rivet head retains its shape. This proposal is disadvantageous in that a not fully polymerised material may have to be stored in refrigerated conditions for considerable periods before use and the heat is also applied in an inefficient and uncontrolled manner.
It is an object of the present invention to provide a blind rivet having a shank made at least partially of heat-softenable polymeric material, the rivet having heating means for softening the shank material to allow formation of a further rivet head in an efficient and controllable manner while the rivet is in situ.